1. Field of the Invention
The present invention relates to reduction of tower shadow effect of a downwind horizontal axis wind turbine.
2. Description of Related Art
Generally, for a downwind horizontal axis wind turbine disposing a rotor downwind to the tower, variable load and ultra-low frequency noise caused by aerodynamic interaction of tower wake and blades have been a technical problem, and are known as the tower shadow effect.
In the past, as a method to suppress this tower shadow effect, a clearance between the rotor and the tower has been secured. This method includes relatively enlarging clearance by making the tower diameter small.
Japanese patent publication JP-Sho-59-147879A (See lower right-hand corner, Page 2) mentions a downwind horizontal axis wind turbine, wherein a airfoil is attached to the tower, structured so as to support the upper and lower parts of the airfoil which is rotatable freely with bearings. In the document, it is disclosed that the tip of the airfoil, which is attached to the tower, always faces downwind, and that the wind against the tower flows smoothly along the airfoil, causing no vortex and little decrease in flow speed behind the tower. Furthermore, therefore, it is disclosed that it prevents excessive bending force on the blades, substantially improves durability, and substantially reduces noise caused by vibration in twisting direction.
However, the method ensuring a clearance between the rotor and the tower is impractical in view of retention of strength and stiffness of tower.
The technique cited in JP-Sho-59-147879A would involve a large-scale apparatus resulting in cost increase. Depending on wind speed, a resonance of the self-excited vibration of such an apparatus and the natural vibration of the tower may occur. In the case of sticking, a considerable load is generated around the maximum lifting capacity and the maximum drag. Difficulty in apparatus maintenance is another problem.